This invention relates to cavity filters and more particularly to an arrangement to compensate for frequency drift due to temperature variations in such cavities.
Temperature dependent frequency drifts from resonant frequency in TEM coaxial cavity filters are usually compensated by constructing the entire cavity, or at least critical areas of the cavity, with low thermal expansion coefficient material. INVAR steel has been successfully used in numerous applications in various filter designs since it has one of the lowest thermal expansion coefficients in the metal family. However, in lower frequency bands (below 2000 megahertz) this practice becomes expensive since the TEM coaxial cavity and resonant structures are machined at greater cost.
Temperature dependent frequency drifts become more severe where the TEM coaxial cavity design requires solid dielectric loading. As an example a dielectric loaded TEM coaxial cavity is employed to avoid multipacting in a space vacuum environment. Such an arrangement is disclosed in the copending application of A. Kivi and L. Feit, Ser. No. 626,161, filed Oct. 28, 1975, whose disclosure is incorporated herein by reference. The solid dielectric, such as REXOLITE No. 1422, has a much higher thermal coefficient of expansion, and when used in a metal cavity its dimensions and tolerances must be compatible with the lower thermal coefficient of expansion of the metal of the cavity and the metal of the coaxial resonator or inductance rod. Therefore, a cavity constructed from a lower thermal coefficient of expansion material would further complicate design efforts.